Cavitation in liquids occurs when the liquid pressure is reduced sufficiently so that vapor bubbles of dissolved gas form. For example, cavitation readily occurs at the tips of rotating propeller blades when the pressure is sufficiently reduced. When cavitation bubbles implode a high-velocity liquid jet is created that can impinge on propellers and cause erosion. Cavitation can also occur in fuel lines feeding jet engines and limit jet engine performance. Cavitation is problematic in turbo-machinery, defense, propulsion, heat transfer, medicine, and a multitude of chemical process applications.
In general, a liquid can be placed into an amorphous metastable state by either superheating or super-tensioning and theoretically can then reach thermodynamic spinoidal limits (the stability limit before a liquid turns to gas) at which time it explosively changes state from liquid to vapor. Such amorphous states can be approached when heterogeneous nucleation sites such as air bubbles, motes or imperfect wetting at solid-liquid interfaces are eliminated or made inactive. This has typically required laborious efforts such as repeated degassing, heat treatment, filtration, use of surfactants and annealing and filtration to remove motes.
Table 1 provides a summary of certain physical properties of liquids including surface tension, vapor pressure, viscosity and an estimate for the critical radius (rc), which is the radius which a cavity in a liquid must attain to permit it to grow into a visible sized bubble. rc is a threshold value for nucleation of a cavitation event. If a liquid cavity has a radius that is smaller than rc, a much larger value of differential pressure, Δp, will be required to cause cavitation. The relationship between rc and Δp is set out below.rc=2σ/Δp, where, Δp is the pressure difference between the pressure of the gas within a cavity and the external pressure of the liquid field and σ is the surface tension of the liquid. rc is also affected by liquid viscosity but that term normally creates a second order effect of much less significance and can effectively be ignored.
TABLE 1Physical properties of various liquidsVaporpressure,rc (nm) (for Δp =ViscosityMaterialσ (mN/m)Pv (kPa)10 bar)(cp)WaterT = 300 K733.56~1400.87T = 358 K6157.90.31T = 458 K411130Acetone (300 K)2333~400.3Dodecane250.02~401.3Ethanol238.7~401.02C2Cl4312.7~571.11Ethanol-Water305.6*~57(40/60) wt. %*from Raoult's law − α × Pethanol + (1 − α) × Pwater
The propensity for cavitation in liquids and liquid systems is dependent on two parameters as shown in the following expression:Cavitation propensity=Minimum (intermolecular bond strength, surface-liquid bond strength)
The surface-liquid interface bond strength refers to the bonding of liquid molecules to surfaces in contact with the liquid. This includes surfaces of container walls, particulate surfaces and dissolved gas. The surface-liquid interface bond strength=inter-molecular attraction+molecular mechanical interlocking+force to overcome 2σ/rc.
Intermolecular strength refers to interactions between the liquid molecules themselves. Intermolecular strength is equal to electron based attraction such as Van der Waal's forces and/or hydrophobic interaction. These parameters, while relatively smaller than the strength of ionic or covalent bonds, can be quite large for liquids such as water which can support a tensile pressure down to about −1,400 bar (or −20,000 psia) and generally range from −200 to −1400 bar. This is substantially below maximum tensile pressure states of around −10 bar in most practical engineering and technological applications. Nucleation theory-based estimates for the ultimate tensile strengths or Pneg, for various liquids (organic and inorganic) can be calculated from the following expression,
            P      _        neg    =      -                                        16            ⁢            π                    3                ⁡                  [                                    σ              3                                      kT              ⁢                                                          ⁢                              In                ⁡                                  (                                      NkT                    ⁢                                          /                                        ⁢                    h                                    )                                                              ]                            1        /        2            where, k and h are the Boltzman and Planck constants, σ is the surface free energy. (i.e., surface tension), N is the number of activated atoms/molecules, and T is the liquid temperature. Because most liquids contain motes and/or microgas bubbles, methods for avoiding cavitation will depend on reducing cavity formation at interfaces between liquid and substrate surfaces and between liquid molecules and impurities. Table 2 presents results of evaluations of ultimate tensile strengths for various liquids based on the above formulation.
TABLE 2Predicted/theoretical & measured negative pressurethresholds for various liquids (T = 300 K)Maximum Tensil (Negative)Pressures (bar)LiquidTheory (Eq. 4)MeasuredWater−1,440−1,400Acetone−243Not availableChloroform−321−317Benzene−348−288
New methods for delaying or preventing cavitation in fluids are needed. They could be used to provide liquid environments that can withstand tension with minimal cavitation. They could be used in studies of fundamental fluid characteristics. To date, high tension levels in macroscale systems have been difficult to achieve due to cavitation. Cavitation resistant liquid could be used to generate tailored bubble cluster fields by triggering bubble nucleation with focused photon beams, such as laser beams, or with ionizing neutrons which could be used in chemical process systems as well as for enhancing mixing caused by bubbling in a space-time basis. Such methods would also be useful to prevent choking in jet fuel lines and could be used in conjunction with vessels, such as submarines, that employ propellers. Such methods would extend propeller life, increase acceleration and velocity characteristics, and reduce the acoustic signal of such vessels.